1. Field of the Invention
This application relates broadly to improvements in automatic backwash filtering methods and systems. More particularly, it concerns methods and apparatus for prewashing of filter cells in automatic backwash filter systems in conjunction with backwash operations in such systems.
2. Description of the Prior Art
Automatic backwash filter systems are known in the art as exemplified in U.S. Pat. Nos. 3,239,061; 4,133,766; 4,617,131 and 4,764,288 currently assigned to Infilco Degremont Inc., the assignee of the invention disclosed herein. Such systems are further exemplified in Infilco Degremont's brochure DB-370, dated May, 1989, entitled ABW.RTM. Automatic Backwash Filter. The disclosures of these patents and the brochure are incorporated herein by reference.
Automatic backwash filter systems are used for the treatment of industrial and municipal water supplies, for treatment of industrial and sewage effluents, etc. because of their proven efficiency and performance for such operations. Such systems are superior to other equipment designed for the same services because they are automatic, self-cleaning and provide optimum performance, without shutdown or changeover typical of conventional filtration equipment.
Automatic backwash filter systems comprise a battery of individual cells that filter influent by gravity flow through them into a common launder. Backwashing is performed automatically and seriatim on the separate cells and in improved types the quality of such backwashing can be monitored to ensure the backwashed cells will output effluent meeting mandated standards (see U.S. Pat. No. 5,147,560).
After a number of repeated automatic backwash cycles, it is necessary to take the system off line to conduct chemical cleaning thereof. In lieu of this, e.g., where environmental regulations prevent typical chemical cleaning, disinfectants are injected into the suction side of the backwash pump when the backwash pump is operating. However, this latter type operation is ineffective and requires large quantities of chemicals because their effectiveness is reduced by the high velocity of the backwash flow. The present invention addresses these problems of prior methods of chemical cleansing of automatic backwash filter systems and provides improvements that eliminate off line downtime and improve effectiveness of chemical usage.